Rule number one of the universe: stay away from black holes. For the most part, these massive blobs of gravity are gentle giants; leave them be, and they’ll do the same to you. Even if objects do happen to invade its personal space (or gravitational influence), they might just end up in orbit around it. But once in a while, an unfortunate star teeters too close to the edge of one of these behemoths and is ripped apart by its gravity, releasing incomprehensible amounts of energy in the form of a high-speed jet that can have dramatic consequences for anyone unlucky enough to stand in its path.
One such incident took place 8.5 billion years ago in a certain faraway galaxy, with scientists only recently detecting the flash released by the event – which, by the way, would have been brighter than 1,000 trillion suns. Thanks to the distance, Earth was not affected; the further away such a jet occurs, the more its light gets dispersed across space. Had it been close to us, our goose would be cooked – literally. But why would black holes – that, by nature, suck everything in – send such a deadly beam out into space? And does this pose any real danger to our planet?
Let’s first take a quick look at what black holes actually are (read my complete overview here). Essentially, they can be summed up by one word: gravity. Regardless of their size – be it miniature black holes the size of a pea, medium-sized ones consisting of the remnants of a dead star, or supermassive giants in the centers of galaxies – these objects are the densest in the universe, with unimaginable amounts of matter crammed into a tiny space; for example, were one to somehow squish Earth down to the point where it becomes a black hole, it would be the size of a ping-pong ball.
As explained by the theory of relativity, gravity is the warping of the so-called fabric of spacetime, which consists of three dimensions of space and one of time (to simplify things, it can be thought of as a stretched-out bedsheet, with objects placed on the bedsheet stretching it downwards and causing smaller objects to fall towards it). Black holes and their immense gravity warp surrounding spacetime to such an extent that not even light – which is massless – can escape.
But not everything falls in right away. About 1% of supermassive black holes possess what is called an accretion disk, consisting of circling gas and dust; it’s very hot and very fast. As explained here, some material falling towards the black hole possesses angular momentum (a way of measuring ‘the rotational momentum of a rotating object or body’), which means that it does not fall towards the hole in a straight line. According to the conservation of this angular momentum, rotational velocity – or how fast the material moves around the black hole – increases the closer it gets to the event horizon (the ‘point of no return’ of the black hole from which light cannot escape); this is much like a figure skater bringing her arms closer to her center of gravity as she spins to gain speed. As a result, the material swirls around the black hole at dizzying speeds, getting faster the closer it gets to the event horizon.
All of that spinning flattens the material into a pancake-like disk, and, as noted here, conservation of angular momentum also dictates that all these particles move in the same general direction. Still, hot and fast particles have a knack for colliding with each other, and this friction heats the disk to temperatures of up to millions of degrees Kelvin (one million Kelvins is about as hot as the sun’s corona). These scorching temperatures also mean that the disks are extremely bright. Unless an object is at a temperature of absolute zero – which is physically impossible – it emits electromagnetic radiation, or light (humans, for example, emit light in the infrared wavelength – heat – but are not energetic enough to glow in visible colors).
Accretion disks, however, are so full of energy that they can emit wavelengths as short as X-rays (shorter wavelengths mean higher energy light). They would also shine in other wavelengths, including the ones we can see; to the human observer, their bluish-white appearance would be quite the blinding sight. Over time, this matter will fall into the hole, but if the monster continues to ‘feed’ on new matter, the disk is sustained.
A sizzling hot frisbee of matter and energy as well as the certain death awaiting anyone who crosses their threshold should be reasons enough to give black holes a wide berth – but it turns out they can shoot at you, too. Understanding this unique weapon – for which the name ‘Death Star’ would be an understatement – requires a closer look at the accretion disk. Within it, you’ll likely find charged particles – ions – such as protons and electrons that have been stripped of their electric counterparts by the heat. As explained here, electric charges in motion create a magnetic field, meaning the particles zooming around in the accretion disk automatically create such a field. As seen with our own planet’s magnetic field that protects Earth’s atmosphere from harmful charged particles from the sun and deep space, these fields have the ability to redirect particles along its boundaries. This technically happens around a black hole, too, but there’s a twist – literally.
Just like stars, planets, and moons, black holes also spin on their own axes. As they go, they twist the magnetic field created by the disk with them, winding it up to create a helix-like structure that extends outwards from the hole’s ‘poles’ perpendicular to the disk (according to a prevalent theory known as the Blandford-Znajek process). As a result, instead of falling into the hole, some particles get tangled up in the field and are shot out into space in focused jets that travel as fast as 99.99% the speed of light, creating a spectacular light show across the spectrum as well as emitting what are among the fastest moving particles in the universe: cosmic rays. Also known as relativistic jets due to their speeds, they can span a million light years from end to end – such as this one, which is over 50 times as big as its host galaxy.
So, where does all the material fueling these cosmic particle beams come from? To answer this question, look into the heart of virtually any galaxy, where you’d typically find a supermassive black hole acting as a galactic nucleus – we’re talking millions to billions the mass of our sun. Most of them are relatively inactive, such as the Milky Way’s very own Sagittarius A*; in these cases, there are barely enough materials around to sustain an accretion disk (which A* has), let alone a jet (which A* does not have). Again, only 1% of supermassive black holes have a disk, and only one in ten disks produce a jet.
But when the nucleus is active, all hell breaks loose. This can occur in younger galaxies where material is not yet completely settled, or galactic mergers and collisions where stars, dust, and gas are sent flying all over the place. A supermassive black hole and its colossal gravity can then vacuum up all that material and surround itself with a big fat accretion disk. Such an extremely active galactic nucleus is known as a quasar, which consists of a supermassive black hole, its accretion disk, and, sometimes, its jet. As explained here, quasars’ luminosities can vary in intensity in periods lasting months to days (and even hours) as the material it consumes fluctuates. These cosmically tiny timeframes allowed scientists to deduce that the structures must be relatively small; since energy must move through each part of the quasar before it is emitted, a quasar that fluctuates (for example) every few weeks cannot be bigger than a few light-weeks across.
Quasars are among the brightest objects in the universe, often outshining the entire galaxy that surrounds them; their accretion disks can, depending on the hole’s spin, convert up to 32% of its matter into energy, resulting in this impressive luminosity (to compare, sun-like stars’ efficiencies come in at just 0.7%). When a quasar’s jet is pointed directly at Earth, they are known as blazars and appear much brighter still. This is due to a phenomenon known as the relativistic Doppler effect; not only does the light coming directly towards us become blueshifted (squashed into shorter wavelengths), but relativistic effects such as time dilation and apparent motion of the source and observer further contribute to the brightness. But while a galactic death beam pointed directly at us sounds harrowing, those known to us are simply too far away to cause any damage (for example, the blazar Markarian 421 – one of the closest to Earth – is 134 million light years away).
Galactic cores are not the only ones who can produce jets. Technically, any black hole – even ‘smaller’ ones caused by the death of a star – lucky enough to be in the company of a living star of lower mass can consume its neighbor, create a disk, and send out a jet, creating a so-called microquasar. This is one example of an X-ray binary system, where one object (the accretor) sucks matter from the other (the donor), though the accretor could also be a neutron star instead of a black hole.
Most of the time, though, supermassive black holes are good neighbors, even allowing stars near the galactic core to orbit it (in fact, measuring these stars is often how scientists infer the existence of a black hole that lacks an accretion disk; a black hole without the shiny disk is, well, black). But on occasion, a star crosses over into a black hole’s lawn and swiftly meets its fate by being torn apart by massive tidal forces in what is called a Tidal Disruption Event (TDE).
One such event, now named AT2022cmc, occurred in February 2022 in a galaxy now 12.4 billion light years away; the light only took 8.5 billion years to reach us because the galaxy was at that distance from Earth when the implosion occurred, and has now traveled even further due to the expansion of the universe. In the jaws of the black hole, the doomed star was ‘squeezed like a toothpaste tube’; the encounter ended up creating a jet (which lasted only days), making the occurrence exceptionally rare. What’s more, the jet was pointed right at us and astronomers were able to detect such an event with an optical telescope for the first time. 8.5 billion light years, however, is pretty far away. What if this happens closer to Earth?
Dwelling in the heart of our ‘middle aged’ galaxy, the Milky Way’s resident supermassive black hole Sagittarius A* grew out of its crazy quasar phase long ago. Unless something unusual happens – like if it happens to consume a star as with AT2022cmc – it is unlikely to randomly become a quasar again. What is slightly less comforting is that one of its poles – the place where the jets would come shooting out – turns out to be pointed pretty much right at us. It is also only 26,000 light years away from us, and some relativistic jets can span millions of light years. That doesn’t bode well.
Still, we’re pretty safe for now – but things could get iffy in another 4.5 billion years or so. That’s the ETA of our biggest galactic neighbor, Andromeda, which is currently en route to merge with our Milky Way; such an event could certainly create galactic turbulence that could stir up some matter for the black hole to suck up – and spew outwards. But chances are we’d be either dead or off planet-hopping by then, as our sun’s increasing brightness will likely have made life on Earth unlivable. In addition, even if we did somehow remain on Earth, such galactic mergers are known for sending stars spinning every which way; who knows if we’d still be in the line of fire by the time A* takes aim.
But if we are… that’s curtains for us. Ionizing radiation like gamma rays would fry our planet (read this for a more complete overview of these rays). In addition, such small wavelengths can tear atoms apart, destroying our atmosphere and damaging the DNA in our cells, causing widespread cancer. Even a ten-second gamma ray hailing from 6,000 light years away can cause enough damage to maim our atmosphere to the point where we would no longer be shielded from UV radiation from the sun, while the rays would also interact with atmospheric nitrogen to create nitrogen dioxide. The latter would drench the planet in acid rain and block out visible sunlight while still letting through the UV rays. In short, even if anything survives the actual impact of the jet (which would last hours at minimum), such an event would effectively sterilize the planet, not to mention wreak havoc on the rest of the solar system.
The good news about these jets, however, is that we can see them and detect where they come from; as of now, those pointed at us seem to be millions of light years away at the very least (on the other hand, if we do happen to be staring down the barrel of a blazar, we won’t know until its light hits us). In addition, these events are very, very rare, with only 0.1% of supermassive black holes resulting in them.
As for other jet-producing systems such as microquasars, they are not only rare, but their jets can only travel across a few light-years. Given that the closest confirmed black hole to Earth, Gaia BH1, sits at 1,566 light years away from us, we’re pretty safe. And though it is likely that more of the giants are lurking in systems closer to Earth, for the moment, we seem to be simply out of range for any cosmic assassination attempts.
Space is gigantic, and we reside in a very boring part of our galaxy’s suburbs. While this can get annoying – what with barely any neighboring star systems close enough to explore, making it very hard to look for other life out there – at least these neighbors appear to be as lame as we are, which is arguably preferable to them sniping at us with black hole-powered space beams.
Good read again. Not an easy subject to comprehend. Advantage of living in a lame neighborhood!